Process for the manufacture of a ballistic-resistant moulded article

ABSTRACT

The invention relates to a process for the manufacture of a ballistic-resistant moulded article in which a stack of monolayers is formed, each monolayer containing unidirectionally oriented reinforcing fibres and at most 30 mass % of a plastic matrix material, the reinforcing fibres being highly-drawn polyethylene fibres, and with the fibre direction in each monolayer being rotated with respect to the fibre direction in an adjacent monolayer, the stack then being compressed at a pressure of more than 25 MPa and a temperature between 125 and 150° C., and the plastic matrix material having a 100% modulus of at least 3 MPa.

This application is the U.S. national phase of international applicationPCT/NL2003/000755 filed 31 Oct. 2003 which designated the U.S. andclaims benefit of NL 1021805, dated 1 Nov. 2002, the entire content ofwhich is hereby incorporated by reference.

The invention relates to a process for the manufacture of aballistic-resistant moulded article in which a stack of monolayers isformed, each monolayer containing unidirectionally oriented reinforcingfibres and at most 30 mass % of a plastic matrix material, thereinforcing fibres being highly-drawn polyethylene fibres, and with thefibre direction in each monolayer being rotated with respect to thefibre direction in an adjacent monolayer, the stack then beingcompressed at elevated temperature at a given pressure.

Such a process is known from EP 0833742 A1. EP 0833742 A1 discloses aprocess for the manufacture of a ballistic-resistant moulded article inwhich a stack is made of monolayers, each monolayer containingunidirectionally oriented reinforcing fibres and at most 30 mass % of astyrene-isoprene-styrene triblock copolymer, the reinforcing fibresbeing highly-drawn polyethylene fibres, and with the fibre direction ineach monolayer being rotated with respect to the fibre direction in anadjacent monolayer, the stack then being compressed at a temperaturebetween 115 and 125° C. and a pressure of at most 16.5 MPa. Sinceballistic protective panels or equipment is often stored, or used, attemperatures above ambient conditions, for instance when applied invehicles, the ballistic protection must be guaranteed also at highertemperatures.

A drawback of the known process is that the specific energy absorption(SEA) for rifle bullets, such as AK47, SS109 or 7.62 NATO Ball, of amoulded article manufactured by it is significantly lower at 80° C. thanat room temperature, so that the protection afforded is in many casesinadequate. The SEA is understood to be the energy absorption uponimpact of a bullet hitting the moulded article at such a velocity thatthe probability of the moulded article stopping the bullet is 50% (V₅₀),divided by the areal density (mass per m²) of the moulded article.

The object of the invention is to provide a process providing mouldedarticles that do not have the aforementioned drawback or have theaforementioned drawback to a lesser extent.

This object is achieved by the plastic matrix material having a modulusof at least 3 MPa and the stack being compressed at a pressure of morethan 25 MPa and a temperature between 125 and 150° C.

The process according to the invention can be used for the manufactureof moulded articles, which with high specific energy absorption at 80°C., so that good ballistic protection is afforded.

In the field of layered ballistic-resistant structures high specificenergy absorption against AK47 bullets is generally understood tocorrespond to a SEA of more than 100 Jm²/kg. The SEA of the mouldedarticle manufactured with the process according to the invention ispreferably higher than 120 Jm²/kg and more preferably higher than 140Jm²/kg.

Here and hereafter good ballistic properties are understood to be inparticular a high SEA against rifle bullets such as AK47, SS109 and 7.62NATO Ball.

The advantage of a high SEA is that fragments having a given velocitycan be stopped by a layered article having a substantially lower arealmass. The areal mass indicates the mass per m² of surface of thearticle, and is also referred to as areal density). A low areal mass isvery important for increasing the wearing comfort, which together withgood protection is the main objective when developing new materials inballistic-resistant clothing. A reduction in mass is also of advantagein case of e.g. vehicle or helicopter armouring.

Within the context of the present application monolayer means a layer ofsubstantially parallel reinforcing fibres embedded in a plastic matrixmaterial. The term matrix material means a material, which holds thefibres together and which wholly or partially encapsulates the fibres.Such monolayers (also called prepregs by one skilled in the art) and themethods of obtaining such a monolayer are disclosed in for instance EP191306 and WO 95/00318 A1. A monolayer may be obtained by orienting aplurality of fibres in coplanar and parallel fashion in one plane, forinstance by pulling a number of fibres or yarns from a fibre bobbinframe over a comb, and impregnating the fibres with the plastic matrixmaterial in a known way before, during or after orienting. In thisprocess, fibres may be used that have previously been coated with apolymer other than the plastic matrix material in order to, forinstance, protect the fibres during handling or in order to obtainbetter adhesion of the fibres onto the plastic of the monolayer.Preferably, uncoated fibres are used.

The plastic matrix material in the process according to the inventionhas a 100% modulus of at least 3 MPa. This is understood to be a secantmodulus measured according to ISO 527 at a strain of 100%.

Suitable matrix materials include thermoplastic and thermosettingmaterials. Preferably, thermoplastics are applied as matrix material,and particularly suitable are those matrices that can be applied as adispersion in water. Examples of suitable polymer materials include:acrylates, polyurethanes, modified polyolefins and ethylene vinylacetate. Preferably, the matrix material contains a polyurethane. Morepreferably, the polyurethane is a polyetherurethane; that is based on apolyetherdiol, since that provides good performance over a widetemperature range. In a special embodiment, the polyurethane orpolyetherurethane is based on aliphatic diisocyanates as this furtherimproves product performance, including its colour stability.

The 100% modulus of the plastic matrix material is at least 3 MPa.Preferably the 100% modulus is at least 5 MPa. The 100% modulus isgenerally lower than 500 MPa.

Impregnation of the reinforcing fibres with the plastic matrix materialcan for instance be effected by applying one or more films of theplastic to the top, bottom or both sides of the plane of the fibres andthen passing these, together with the fibres, through heated pressurerolls. Preferably, however, the fibres, after being oriented in parallelfashion in one plane, are coated with an amount of a liquid substancecontaining the plastic matrix material of the monolayer. The advantageof this is that more rapid and better impregnation of the fibres isachieved. The liquid substance may be for example a solution, adispersion or a melt of the plastic. If a solution or a dispersion ofthe plastic is used in the manufacture of the monolayer, the processalso comprises evaporating the solvent or dispersant.

In the process according to the invention the stack is compressed at apressure of more than 25 MPa, in a press or compression-mouldingmachine. Preferably, the pressure is at least 27, or at least 29 MPasince this further enhances performance of the moulded article. Thetemperature during the compression is between 125 and 150° C. A highertemperature has the advantage that the time of compression can bereduced, but the temperature should not exceed 150° C., that is remainbelow the melting range of the polyethylene fibres. In a preferredembodiment, the stack preferably comprising a polyurethane matrixmaterial, is compressed for at least 60 minutes at a temperature between125 and 135° C., or even more preferred for 20 minutes at a temperaturebetween 135 and 150° C.

After pressing at elevated temperature, the stack is cooled beforeremoving from the press to a temperature below 100° C., preferably below80° C. In a preferred embodiment, the stack is cooled while still underpressure, preferably of at least 5 MPa, more preferably under the samepressure as in the preceding pressing step.

The fibre direction in each monolayer in the ballistic-resistant mouldedarticle prepared according to the process according to the invention isrotated with respect to the fibre direction in an adjacent monolayer.Good results are achieved when this rotation amounts to at least 45degrees. Preferably, this rotation amounts to approximately 90 degrees.Such construction is hereafter referred to as being “cross-layered”.

Reinforcing fibre here means an elongate body whose length dimension isgreater than the transverse dimensions of width and thickness. The termreinforcing fibre includes a monofilament, a multifilament yarn, a tape,a strip, a thread, a staple fibre yarn and other elongate objects havinga regular or irregular cross-section.

Preferably, the reinforcing fibres predominantly contain highly-drawnfibres of high molar mass linear polyethylene. High molar mass isunderstood to be a molar mass of at least 400,000 g/mol.

Linear polyethylene is here understood to be polyethylene with less thanone side chain per 100 carbon atoms and preferably less than one sidechain per 300 carbon atoms; a side chain, also called branch, containingat least 10 carbon atoms. In addition the polyethylene may contain up to5 mol % of one of more other alkenes which are copolymerizabletherewith, such as propylene, butene, pentene, 4-methylpentene, octene.

Preferably, use is made of highly-drawn polyethylene fibres comprisingpolyethylene filaments prepared by a gel spinning process as describedin for example GB 2042414 A, GB 2051667 A, or WO01/73173 A1. Thisprocess essentially comprises the preparation of a solution of apolyolefin of high intrinsic viscosity, spinning the solution intofilaments at a temperature above the dissolving temperature, coolingdown the filaments to below the gelling temperature so that gellingoccurs and drawing the filaments before, during or after removal of thesolvent. The shape of the cross-section of the filaments can be chosenthrough selection of the shape of the spinning aperture.

Preferably, the monolayer contains strong polyethylene fibres, with adenier per filament (dpf) greater than or equal to 0.5 dpf. Mostpreferably, use is made of multifilament yarns of ultra-high molar masslinear polyethylene with an intrinsic viscosity of at least 5 dl/g,determined in decalin at 135° C., and a yarn titre of at least 50denier, which yarn has a tensile strength of at least 35 cN/dtex and atensile modulus of at least 1000 cN/dtex and with the filaments having across-section aspect ratio of at most 3. The use of these fibres hasbeen found to improve the high level of protection of theballistic-resistant moulded article manufactured according to theprocess of the invention still further.

In the process according to the invention the stack may be made startingfrom separate monolayers. Separate monolayers are difficult to handle,however, in that they easily tear in the fibre direction. It istherefore preferred to make the stack from consolidated monolayerpackages containing from 2 to 8, as a rule 2, 4 or 8, monolayers thatare placed at an angle with respect to the fibre direction, e.g.cross-wise. Consolidated is intended to mean that the monolayers arefirmly attached to one another. These monolayer packages can be madewith different methods, for example by calendaring between rolls, or bycompression moulding. Very good results are achieved if also themonolayer packages are compressed at an elevated temperature, optionallyat high pressure as in the process according to the invention, andsubsequently cooled under a high pressure; preferably of at least 5 MPa,even more preferably under the same pressure as during the previousstep.

The invention also relates to a ballistic-resistant moulded articlecomprising a stack of monolayers, each monolayer containingunidirectionally oriented reinforcing fibres and at most 30 mass % of aplastic matrix material, the reinforcing fibres being highly-drawnpolyethylene fibres, and with the fibre direction in each monolayerbeing rotated with respect to the fibre direction in an adjacentmonolayer, characterized in that the plastic matrix material containspolyurethane and the moulded article has an SEA at 80° C. that is atleast 100 J/(kg/m²).

It has, surprisingly, been found that the moulded articles according tothe invention have low acoustic damping. The acoustic damping of mouldedarticles with a thickness of 2 cm, measured at 0.5 MHz, is lower than 30dB. The absorption per unit thickness therefore is less than 15 dB/cm.As a result, the moulded articles can easily be distinguished frommoulded articles according to the state of the art.

EXAMPLES I-XIII

Materials Used

A monolayer package consisting of two monolayers disposed cross-wise atan angle of 90 degrees. The fibres are highly-drawn fibres of high molarmass linear polyethylene of make Dyneema® SK76 with a strength of about36 cN/dtex, a modulus of about 1180 cN/dtex and a fineness of about 2denier per filament with a cross-section aspect ratio of about 1. Themonolayer contains 18 mass % matrix material consisting of polyurethanefrom Baxenden Chemicals Ltd., which polyurethane is based onpolyetherdiol and aliphatic diisocyanate, and is applied as an aqueousdispersion. The 100% strain modulus of the matrix is 6 MPa, as measuredon a film made from the dispersion. The areal density of the monolayerpackage is 130.5 g/m². In Table 1 such materials are indicated by HB25.

Procedure

Short compression cycle: 144 of the above-mentioned monolayer packageswere stacked to yield a package whereupon the package in its entiretywas preheated in an oven for 2.5 hours at a temperature of 125° C. Thepackage was then compressed for 10 minutes in a press at the temperatureand pressure given in Table 1. The package was subsequently cooled to atemperature of 60° C. under the same compression pressure.

Long compression cycle: 144 of the above-mentioned monolayer packageswere stacked to yield a package and subsequently compressed for 65minutes in a press at the temperature and pressure given in Table 1. Thepackage was subsequently cooled to a temperature of 60° C. under thesame compression pressure.

Test Procedures

The modulus of the matrix material was determined according to ISO 527.The 100% modulus was determined on film strips with a length of 100 mm(free length between the clamps) and a width of 24 mm. The 100% modulusis the secant modulus measured between strains of 0% and 100%.

The V₅₀ of the panels was measured according to a procedure derived fromStanag 2920. The panels were clamped onto a steel frame and fired at,without backing, with AK47 ammunition at 20 and 80° C. The panels wereconditioned at a controlled temperature in an oven for at least 24 hoursprior to the test. Immediately before the test started, the conditionedpanel to be fired at was taken from the oven and attached to the frame,upon which firing took place within 2 minutes.

The method used for determination of the acoustic damping is a pulsetransmission measurement technique (frequency range 0.5-10 MHz).Measuring took place at a frequency of 0.5 MHz. The panels used fordetermination of the acoustic damping have an areal density of about 19kg/m² and a thickness of about 20 mm. The samples were strips having awidth of about 100 mm that had been cut from (the side of) a panel.

The transmission was measured between a transmitter and a receiver at adistance of 10 cm on both sides of the sample and the acoustic couplingwas realized by means of a water jet. The entire surface area of thesamples was scanned, after which the average damping was determined.

Results

Table 1 presents the V₅₀ values and SEA values obtained as a function ofthe compression temperature and pressure. Time of conditioning at 80° C.was 24 hours, except for examples IV and VIII (1 week), and V (4 weeks).For some panels also acoustic damping results are given.

Comparative Experiments

The process described under the Examples was repeated at compressionpressure of less than 25 MPa and a compression temperature of 125° C.The results of this Comparative Experiment A are given in Table 1.

The above process was repeated for a monolayer package consisting offour monolayers disposed cross-wise at an angle of 90 degrees. Thefibres are high-drawn fibres of high molar mass linear polyethylene ofmake Dyneema® SK76 with a strength of about 36 cN/dtex, a modulus of1180 cN/dtex and a fineness of 2 denier per filament with across-section aspect ratio of about 1. The monolayer contains about 18mass % matrix material consisting of Kraton®, applied from an aqueousdispersion. Kraton is a styrene-isoprene-styrene triblock copolymercomposition. The 100% strain modulus of this matrix is 1.4 MPa. Theareal density of the monolayer package is 265 g/m². In Table 1 thismaterial is indicated by HB2 for Comp. Exp. B-D. The areal density ofthese samples was about 19 kg/m², as for all others.

The results show that the SEA at 80° C. against AK47 of the mouldedarticles manufactured according to the process of the invention isalways higher than 100 J/(kg/m²).

TABLE 1 Compression Compression Compression V₅₀ SEA Conditioning V₅₀ SEADamping time pressure temperature 20° C. 20° C. time at 80° C. 80° C.80° C. 05. MHz Material minutes MPa ° C. m/s J/(kg/m²) days m/sJ/(kg/m²) dB/cm I HB25 10 30 125 833 148 1 776 128 14.9 II HB25 10 30135 878 164 1 778 129 III HB25 10 30 140 836 149 1 817 142 IV HB25 10 30140 780 129 7 776 128 V HB25 10 30 140 828 146 28 775 128 VI HB25 10 30140 765 125 1 792 133 17.3 VII HB25 10 30 145 1 845 152 VIII HB25 10 30145 7 826 145 IX HB25 65 30 125 813 141 1 773 127 X HB25 65 30 140 830146 1 812 140 A HB25 10 16.5 125 743 117 1 605 78 25.5 B HB2 65 30 140835 148 1 <618 <81 >40 C HB2 65 16.5 125 848 153 1 <631 <85 D HB2 1016.5 125 807 139 1 <631 <85 >40

1. Process for the manufacture of ballistic-resistant moulded articlecomprising the steps of forming a stack of monolayers in which eachmonolayer contains unidirectionally oriented reinforcing fibres and atmost 30 mass % of a polyurethane matrix material having a 100% modulusof at least 3 MPa, the reinforcing fibres being highly-drawnpolyethylene fibres, wherein said step of forming the stack ofmonolayers includes rotating the fibre direction in each monolayer withrespect to the fibre direction in an adjacent monolayer, and thereaftercompressing the stack of monolayers at an elevated temperature between125 and 150° C. and at a compression pressure of more than 25 MPa. 2.Process according to claim 1, wherein the step of compressing the stackof monolayers is practiced by compressing the stack for at least 60minutes at a temperature between 125 and 135° C.
 3. Process according toclaim 1, wherein the step of compressing the stack of monolayers ispracticed by compressing the stack for 20 minutes at a temperaturebetween 135 and 150° C.
 4. Ballistic-resistant moulded articlecomprising a stack of monolayers, each monolayer containingunidirectionally oriented reinforcing fibers and at most 30 mass % of apolyurethane matrix material having a 100% modulus of at least 3 MPa,the reinforcing fibres being highly-drawn polyethylene fibres, and withthe fibre direction in each monolayer being rotated with respect to thefibre direction in an adjacent monolayer, wherein the moulded articlehas an SEA at 80° against AK47 bullets that is at least 100 J/(kg/m²).5. Ballistic-resistant moulded article according to claim 4, with anacoustic damping, measured at 0.5 MHz, of less than 20 dB/cm.